FGF23 and hypophosphatemia: A delay in time between onset of symptoms and a further delay in localization and resection of the FGF23-producing tumor may be associated with considerable morbidity because of progressive osteomalacia if the hypophosphatemia remains untreated or is more difficult to control if treated with active vitamin D metabolites as the tumor growth increases production of FGF23, resulting in increasing renal phosphate wasting.